Of 375 patients with prostatic carcinoma treated definitively with radiation therapy at this institution with at least a 5 year follow-up, 23 patients failed locally only, 72 failed with distant metastasis only, 60 had both local and distant failure, while 220 showed no evidence of disease. In search for a possible marker for local failure following radiation therapy, we examined several nuclear morphometric parameters which have been shown to correlate with the biologic aggressiveness of this disease. The 23 locally failed only patients were matched with 23 no evidence of disease patients for stage, grade, treatment modality, prior surgery, age at diagnosis and race. Archival hematoxylin and eosin slides were obtained for 22 of the 23 matched pairs, and morphometric features, including nuclear roundness factor and nuclear area, as well as numbers of nucleoli were assessed using computer-assisted image analysis in both tumor cells and normal prostatic epithelium. Tumor nuclei from the locally failed only patients had significantly higher nuclear roundness factor values (p = 0.0089) compared with tumor cells from no evidence of disease patients. Analysis of these data by clinical stage demonstrated no significant differences between the locally failed only and no evidence of disease patients. Likewise, there were no significant differences in nuclear roundness factor values of locally failed only and no evidence of disease patients with poorly or moderately well-differentiated tumors. However, there was a highly significant difference (p = 0.0012) in the nuclear roundness factor values of locally failed only and no evidence of disease patients with well-differentiated tumors. Thus, there appears to be a subset of patients with well-differentiated adenocarcinoma of the prostate who have significantly more irregular tumor nuclei and who fail locally only following definitive radiation therapy.